There are no clear estimates of numbers of displaced, but Kurdish sources say at least tens of thousands have left their homes.

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Kurdish authorities accused Turkey of shelling a prison holding Islamic State (IS) group prisoners in Qamishli in the east of the border region in a "clear attempt" to help them escape.

Kurdish authorities have called for a general mobilisation and urged people to "head to the border with Turkey… to resist in this sensitive, historic moment".

What resistance can the Kurdish-led forces offer?

The Kurdish-led Syrian Democratic Forces (SDF) currently number about 40,000 fighters, with tens of thousands of others in parallel Kurdish security services, Kurdish sources say.

The US joint task force on operations against IS in Iraq and Syria describes them as "tenacious fighters with a degree of basic military training to function as infantrymen".

But they are deficient in heavy weaponry that could be used against tanks or aircraft, though some units may have anti-tank missiles.

In operations against IS, they relied on close coalition air support but in the flat, open country of Syria's northern border they will be vulnerable to air and artillery attack.

Transactional alliances

Even by President Trump's own remarkable standards his off-the-cuff remark that the US alliance with the Kurds is of little importance because they were not at Normandy, ie they did not fight with the US and its allies in World War Two, is extraordinary.

For Mr Trump alliances are simply transactional – business arrangements to be judged according to a brutal and short-term cost benefit analysis. What is the US giving and what is it getting in return?

In seemingly writing off the Kurds he suggests that the US can easily find other allies in the region. Really? Has he already forgotten recent history? The Kurds were the only capable and reliable local ally in the struggle against IS.

But what will Mr Trump do about Turkey who, incidentally, were not at Normandy either? This is fast becoming a major test of Turkey's standing within Nato, with many fearing it has become a far from reliable ally of the West.

Why has Turkey attacked?

Turkey says the aim of the operation is to "prevent the creation of a terror corridor" on the border and create a "safe zone" cleared of Kurdish militias which will also house two million Syrian refugees, nearly half those currently living in Turkey.

But critics say the operation could lead to ethnic cleansing of the local Kurdish population in northern Syria, potentially displacing 300,000 people, and a revival of IS.

Turkey considers the Kurdish YPG militia – the dominant force in the SDF – an extension of the banned Kurdistan Workers' Party, whiRead More – Source